The present invention relates to a control drive system for a vehicle operating between two points and, more particularly, to a control drive system wherein AC is converted to DC aboard the vehicle for energizing the drive means of the vehicle.
In mining operations it is common to employ a shuttle car for transporting the material being mined, for example, coal, from the work area to an unloading area where the material is unloaded onto a conveyor belt or onto cars for transporting out of the mine. Such cars are electrically operated with the electrical energy being supplied thereto from a source at the unloading point via a cable which is reeled out from a reel as the car leaves the unloading point toward the work area, and conversely is reeled onto the reel as the car leaves the work area and returns to the unloading point. Two types of systems are presently in common usage, namely, the AC type and the DC type. In the AC type, alternating current supplied at the unloading point is applied via a trailing cable to the shuttle car and is utilized for driving AC motors which supply the driving power for the wheels of the shuttle car. The AC type of system has a number of disadvantages inherent in the use of AC drive motors. Each time the shuttle car is stopped for loading, unloading, or other reasons, the motors are de-energized and must be started from the de-energized condition. In-rush current for AC motors is very high, for example, in a 480 volt, three phase AC machine, the in-rush current may be as high as 290 amperes. This causes a large voltage drop in the trailing cable, resulting in high energy losses in the cable and thus greatly decreasing the efficiency of the system. To compensate for these losses, it becomes necessary to employ relatively large cables so that the motors may be operated at their rated voltages. Another disadvantage of the AC system is the problem of running over synchronous speeds wherein the AC motor acts as a generator and may introduce excessive current in other drive motors of the system. For example, when one AC motor is used to drive wheels on one side of the shuttle car, and another motor is utilized to drive the wheels on the other side of the car, as the car goes around a corner the motor driving the outside wheels will be required to go above synchronous speed, which may cause excessive current to be applied to the motor driving the inside wheels.
The DC type of system utilizes DC supplied from a DC source via a trailing cable to the car. DC may be supplied from outside of the mine, or may be converted from alternating current supplied at the unloading point by rectification. It is then supplied via a DC cable to the shuttle car wherein it is utilized for supplying DC traction motors for driving the wheels of the car. The use of DC traction motors has the advantage of providing excellent performance, especially under high load conditions. However, as in the AC case, serious cable losses result at the start-up due to in-rush current. For example, a 250 volt DC machine may have an in-rush current of 250 amperes. Thus a relatively large DC cable must be used in order to compensate for the voltage drop during the start-up and tramming operation. Another disadvantage of the DC system is that only DC is available aboard the car, where it would be highly desirable to utilize AC for auxiliary equipment, such as, a pump motor for the hydraulic system aboard the vehicle, or a conveyor motor for the operation of the unloading conveyor aboard the vehicle. For these purposes it is desirable to utilize AC motors operative at a constant speed and which are highly reliable. Another disadvantage of the DC system is the potential damage which may result to the diodes of the rectifier employed to convert the AC to DC at the unloading point due to faults. The most common place for faults to occur is in the trailing cable, and at the occurrence of such a fault, high currents may be applied to the diodes of the rectifier which is located adjacent to the AC source at the unloading point.
The AC-DC system of the present invention takes advantage of all the inherently desirable features of the DC system while introducing none of the disadvantages of either the DC or AC system. In the system of the present invention, AC supplied from the unloading point is converted to DC by rectification performed aboard the shuttle car itself. The DC is then controllably applied to drive DC traction motors aboard the car. However, in that AC is available aboard the shuttle car, auxiliary motors, such as, pump and conveyor drive motors, may be AC motors which provide constant speed and high reliability of operation. Due to the AC-DC conversion aboard the car, in-rush current losses are greatly minimized as compared to the AC type or DC type of system. For example, an in-rush current of 60 amperes may be expected as compared to a 250 to 290 amperes in-rush current for the AC and DC type systems, respectively, for motors of the same ratings. Hence, much smaller trailing cables may be utilized for the AC-DC system of the present invention. Additionally, due to lower cable losses a higher over-all efficiency is obtained in the present system. In that the diodes used for rectification of the AC to DC aboard the shuttle car are separated from the AC Source at the unloading point by the cable, any fault currents induced by a fault in the cable will be attenuated by the impedance of the cable, thereby diminishing the possibility of damaging diodes.
Broadly, the present invention provides a control drive system for supplying a vehicle operating between a base point and a work point, wherein AC at the base point is supplied to the vehicle via a cable and is converted to DC aboard the vehicle for driving the vehicle.